This invention relates to electric switches for controlling power applied to a load wherein a heat sink is provided externally to the apparatus and has a power switching semiconductor of the apparatus attached thereto. More particularly, the invention relates to electric switches for controlling a motor of a portable electric tool. Still more particularly, the invention relates to electric switches for controlling the speed of a DC motor of a portable electric tool and to battery clip terminations of the electric switch to a battery supply.
Electric switches for controlling portable electric tools are provided in a large number of physical and functional variations due to diverse customer requirements. On the other hand, the overall package size and configuration and mounting features for securing the switch within the tool housing are somewhat standardized. To realize any economy of scale advantage in manufacturing switches to numerous variations, it is necessary to achieve as much commonality in parts as possible from one variation to another.
Electric switches of the foregoing type for controlling the speed of AC motors have long been available. Recently, battery powered DC tools have become prevalent. The switches for controlling the speed of DC motors in battery powered tools are required to have essentially the same configuration and general operating principles as the switches for controlling AC motors. Speed control switches tend to be compact, densely arranged with components and incorporate circuitry including a power switching semiconductor which generates significant heat that must be dissipated to prevent damage to the control and to the tool. Heat sink requirements often exceed the available space within the switch to the end that the semiconductor switching element is brought outside of the switch and attached to a portion of the tool housing, utilizing the tool housing as a heat sink. The control of DC motors for portable tools is essentially similar to that for AC controls, but discrete differences in the DC control generate greater heat to be dissipated to the extent that direct use of the tool housing as a heat sink is not feasible.
U.S. Pat. No. 4,719,395 issued Jan. 12, 1988 to Tatsuo Aoi et al discloses a DC speed control switch wherein a cover assembly of the switch housing per se is a heat sink. The cover assembly comprises an interior metal cover which attaches to the insulating switch base with a snap fit to overlie a printed circuit board and other electrical switch components disposed within the switch base. The power switching transistor is sandwiched between the interior metal cover and an exterior metal cover, clamped tightly therebetween by a screw. The exterior cover envelopes the interior cover and also snap attaches to the insulating switch base. This double formed cover arrangement, clamped around a semiconductor and each cover snapped to an insulating base, is complex and costly. Electrically conductive metal is used as a housing element as contrasted to insulative plastic materials prevalent in current technology to provide a fully insulated electrical switch housing. The particular cover arrangement of the aforementioned patent provides external heat sinking for the power switching semiconductor, but still locates the power switching semiconductor directly above the switch base cavity in close proximity to the printed circuit board and the electrical components of the switch.
Another concern in providing speed control switches for DC electric motors is the connection between the switch and the power source. In some devices, wires are utilized between the switch and a battery to provide a permanent connection. In other versions, a battery or a battery module is removable from the tool and the connection between the battery and the switch is broken upon removal. This connection is customarily a battery clip made a part of the switch. It is preferable and advantageous that the battery clip be an add-on structure which can readily convert a hard wire termination of a basic switch to a battery clip termination. U.S. Pat. No. 4,586,777 issued May 6, 1986 to Julius P. Wied discloses a battery clip adaptor which is snap-fit to a switch module. This adaptor comprises a plastic insulating base 10 to which are mounted separate pairs of clips and terminal prongs secured together and to the base by respective rivets the prongs extending into electrical contact with power terminals of the speed control switch when the adaptor base is snap-fit attached to the switch housing. The numerous discrete parts and riveting operations required to assemble the adaptor introduce a significant cost burden to the switch.
It is an object of this invention to provide a fully insulated speed control switch for a DC electric motor having an external heat sink attached around the insulating housing of the switch, or into which such insulating housing is nested, and to which a power switching semiconductor is directly attached.
It is a further object of this invention to provide an electric speed control switch for a DC motor having an external heat sink attached to an insulating housing of the switch and utilizing the heat sink as a clamping member for affixing battery clip terminals to the switch.